Berkeley
- Geckos are able to scurry up walls and across ceilings thanks
to two million microscopic hairs on their toes that glom onto
surfaces in a way that has given engineers an idea for a novel
synthetic adhesive that is both dry and self-cleaning.

In a paper
in this week's issue of Nature, University of California,
Berkeley, biologist Robert J. Full, Lewis and Clark College
biologist Kellar Autumn and their colleagues report the first
measurement of the forces that these hairs, or setae (see'
tee), exert on a surface.

Working
with the "Cadillac" of the gecko world, a Tokay
gecko (Gekko gecko) native to Southeast Asia, the team of
biologists and engineers showed that the combined adhesive
force of all the tiny hairs lining the gecko's toes is 10
times greater than the maximum force reportedly needed to
pull a live gecko off the wall. Geckos apparently use only
a fraction of the hairs at one time, though they have been
known to hang from the ceiling by one toe.

The key
seems to be the hundreds to thousands of tiny pads at the
tip of each hair. These pads, called spatulae, measure only
about ten millionths of an inch across. Yet, they get so close
to the surface that weak interactions between molecules in
the pad and molecules in the surface become significant. The
combined attraction of a billion pads is a thousand times
more than the gecko needs to hang on the wall.

"These
billion spatulae, which look like broccoli on the tips of
the hairs, are outstanding adhesives," said Full, a professor
of integrative biology and head of the Poly-PEDAL (Performance,
Energetics, Dynamics, Animal Locomotion) Laboratory at UC
Berkeley. "Geckos have developed an amazing way of walking
that rolls these hairs onto the surface, and then peels them
off again, just like tape. But it's better than tape."

"Getting
yourself to stick isn't really that difficult, it's getting
off that is the problem," noted Autumn, a former postdoctoral
student in Full's laboratory who now is an assistant professor
of biology at Lewis and Clark College in Portland. "When
a gecko runs it has to attach and detach its feet 15 times
a second. We think we know how it does that so rapidly."

In order
to measure the extremely tiny forces involved when one hair
sticks to a surface, Full and Autumn teamed up with two engineers,
Ron Fearing at UC Berkeley and Thomas Kenny of Stanford University.

Kenny micro-machined
a minuscule device to measure the forces involved in attaching
to a surface, while Fearing employed a fine aluminum wire
to measure the forces when detaching.

With Kenny's
MEMS (microelectromechanical system) tool, Full, Autumn and
their colleagues showed how touching the end of a hair to
a surface is not good enough - the hair slips right off. If
the gecko instead engages the surface by pushing in and pulling
slightly downward, it can achieve 600 times greater sticking
power than friction alone could account for. Full suspects
that the uncurling motion as the gecko attaches its foot to
a surface does this automatically.

Using Fearing's
device, they also showed that pulling away is not enough to
disengage. The strength of attachment is so strong that a
single gecko hair, only one-tenth the diameter of a human
hair, could bend the aluminum wire. In fact, Autumn said,
a single hair could lift an ant, while a million hairs covering
an area the size of a dime could lift a small child of about
45 pounds.

If the
hair is levered upward at a 30 degree angle, however, the
spatulae at the end of the hair easily detach. The gecko does
this simply by peeling its toes off the surface.

"It's
quite challenging to make these measurements," Full said.
"The integration of interdisciplinary ideas really made
this project work."

To explain
the ability of geckos to rapidly run up a vertical surface
and even stick to the ceiling, Full, Autumn and their colleagues
considered and ruled out the most common ways animals use
to stick to a surface.

They calculated
that suction is much less effective than the measured sticking
force of geckos. Plus, geckos can stick to the wall in a vacuum.
Also, there is no evidence that geckos use glue: there are
no glue glands on the foot, nor is glue residue left on the
surface. The hairs do not interlock with the surface, as with
Velcro, nor is friction likely: friction could not explain
their ability to walk on the ceiling. Electrostatic attraction
was ruled out by other researchers.

The most
likely explanation, Full said, is intermolecular forces so
weak that they are normally swamped by the many stronger forces
in nature. Intermolecular forces come into play, though, because
the gecko foot hairs get so close to the surface.

"The
hairs allow the billion spatulae to come into intimate contact
with the surface, combining to create a strong adhesive force,"
Full said. "Our calculations show that van der Waals
forces could explain the adhesion, though we can't rule out
water adsorption or some other types of water interaction."

Van der
Waals forces are among several types of intermolecular forces
that are weak until surfaces get very close. When a large
area is in contact, though, they can add up to a strong attraction.
Van der Waals forces are responsible for the attraction between
layers of graphite, for example, and the attraction between
enzymes and their substrate.

Van der
Waals forces arise when unbalanced electrical charges around
molecules attract one another. Though the charges are always
fluctuating and even reversing direction, the net effect is
to draw two molecules together, such as molecules in a gecko
foot and molecules in a smooth wall.

Though
the setae work extremely well in adhering to a smooth surface
such as glass, Autumn said that in the natural world, waxy
coatings on leaves may hinder adhesion by resisting the intermolecular
interactions. Therefore, while in the laboratory geckos may
not need more than 10 percent of their setae to stick to glass,
they may need to use more to walk around on vegetation.

Since the
hairs and spatulae work so well, Fearing and Kenny have launched
an effort to make artificial hairs that use the same sticking
technique and could make a strong yet dry adhesive. In a yet-to-be-published
paper, Autumn and Full report, too, that the gecko hairs are
self-cleaning, unlike any other known adhesive.

"We
clogged their hairs with microspheres, and five steps later
they were clean," Full said. "We don't know why,
but it's amazing."

The next
step, Full and Autumn said, is to find a way to study individual
spatulae and measure their attractive force.

They also
want to study other creatures that use the same technique,
but with somewhat different equipment. Lizards like the anole
and the skink use foot hairs of different shapes to walk on
vertical surfaces, while even the blood-sucking insect known
as the kissing bug has apparently evolved foot hairs with
a single spatula at the tip.

Full and
Autumn also have an association with IS Robotics, Inc., that
has resulted in a mechanical gecko that adopts the peeling
action of geckos to walk on vertical surfaces. At the moment,
the small robot uses an adhesive glue to stick to the wall.
But Full hopes that an artificial dry adhesive will yield
a much better and longer lasting robot.